Photoelectric controlled line following devices as used with oxygen jet cutting machines



Nov. 13, 1962 L. J. HANCOCK 3,063,698

PHOTOELECTRIC CONTROLLED LINE FOLLOWING DEVICES AS USED WITH OXYGEN JET CUTTING MACHINES Filed June 1, 1960 2 Sheets-Sheet 1 /9 /9a gm /4 4 Hrll! Inventor LEsLl E J. HANC ocK By g Attorney Nov. 13, 1962 L. J. HANCOCK 3,063,698

PHOTOELECTRIC CONTROLLED LINE FOLLOWING DEVICES AS USED WITH OXYGEN JET CUTTING MACHINES FiledJune 1, 1960 2 Sheets-Sheet 2 1n venlor By A:

LESLI'E J'- HANCOCK torney United States Patent() PHOTQELECTRIC CONTROLLED LINE FOLLOW- ENG DEVICES AS USED WITH OXYGEN JET CUTTING MACHENES Leslie John Hancock, Puriey, England, assignor to Hancock & Co. (Engineers) Limited, Croydon, England, a British company Filed June 1, 1960, Ser. No. 33,192 Claims priority, application Great Britain June 1, 1959 7 Claims. (Cl. 266-23) This invention relates to photoelectrically controlled outline or edge following devices, and more particularly to a photoelectric following device which follows an outline, or the edge of an outline or area, to control the steering of a movable cutter in an oyxgen jet cutting machine, the following device turning on its axis, which is substantially perpendicular to the plane of the outline, to an extent depending upon the deviation of the outline from the straight line which is instantly being followed by the following device.

Following devices now in use in oxygen jet cutting machines rely either on the amount of light reflected from a drawing in which the outline is in the form of a thick black line, the following device always adjusting itself so that a projected spot of light is half over the line, or on the scanning of a short path transverse to the line, for example, by the use of a vibrating photoelectric element, in which the following device sets itself so that an equal amount of the White background is viewed on either side of the line image.

In order that the cotnrol mechanism for the machine may be properly directed the portion of the line projected on to or scanned by the photoelectric device must be in advance in relation to the control mechanism in order to allow time for the following device to impose the required turning effort on the steering mechanism, and for the latter to respond, when a deviation is required.

The invention may be used in conjunction with line following devices of known types and to facilitate a full understanding of the invention a typical prior art type will be described. This comprises a casing containing a vertical shaft rotatable about its axis and having a tracer wheel mounted on a horizontal axis at its lowerv end. The tracer wheel is driven by a motor running at a substantially constant speed and bears upon a table on which a drawing may be placed containing an outline of the shape of the piece to be cut. The machine includes a lower carriage running on a pair of horizontal parallel rails and an upper carriage running on a second pair of horizontal parallel rails placed at right angles to the first pair of rails. The upper carriage may move in all directions in a horizontal plane and the afore mentioned casing, as well as a flame-cutting torch, are attached to it. Mounted on the upper carriage adjacent the casing, or within the casing itself, is a photoelectric tracing device so ararnged that it looks downward at the drawing on the tracing table. The photoelectric tracer is capable of being rotated about a vertical axis parallel to the vertical axis of the shaft carrying the tracer wheel. This device and the shaft carrying the tracer wheel are coupled together so that they rotate in synchronism and they are adapted to be rotated in either direction by a steering motor which will be more particularly described later.

The tracing device is provided with a vibratory spring arm which carries a photoelectric cell. A lamphouse containing a lamp and lens projects a beam of light downwardly on to the table, so that a small circular area is brightly illuminated. The machine is adjusted so that the outline to be traced lies within this circle, the photoelectric cell is vibrated on a line transverse to the direc- 3,963,598 l ar tented Nov. 13, 1962 tion of the outline so that it alternately looks at one side and the other side of the outline. It is provided with a small aperture so that it only scans a small area at one time.

As the photoelectric cell moves from its central position to one side of the outline it develops a signal which rises from a level corresponding to the dark area of the outline and this signal increases as the photoelectric cell moves until it reaches a maximum when the photo electric cell is looking entirely at the brightly illuminated area at the side of the outline. As it swings back to the centre the signal diminishes to its original low level and after it has crossed the outline the signal rises again to a maximum and thereafter again falls to zero as the cell returns to its central positon. It is convenient to vibrate the arm carrying the photoelectric cell at the mains frequency and a little consideration will show that the photoelectric cell produces a signal which rises from a low level to a high level and back to zero twice in each cycle of the mains frequency; for example, if the mains frequency is 60 c./s. then the signal from the photoelectric cell consists of 120 unidirectional impulses per second. In fact, the output signal from the photoelectric cell is similar to the voltage wave which is obtained by rectifying the mains frequency through a full wave rectifier.

The output signal from the photoelectric cell is passed to an amplifier and an amplified signal is obtained.

The steering motor previously referred to is a well known type of two-phase servo motor in which the field is supplied from the supply mains through a reactor which produces a phase displacement. If a voltage which is in phase with the mains supply is applied to the rotor of the servo motor then it runs in one direction but if a voltage which is in phase opposition (i.e. phase displaced by 180) with the mains frequency is applied to the rotor then the motor runs in the opposite direction. It will be evident that alternate impulses supplied by the amplifier are in phase with the mains supply while the intervening impulses are in phase opposition with the mains supply. If the outline is in the centre of the transverse scanning line of the photoelectric cell the impulses are of equal magnitude and if the amplifier output is applied to the rotor of the steering motor the steering motor will receive a series of impulses, alternate ones of which tend to make it run in one direction and the intervening ones of which tend to make it run in the other direction. Since its inertia, and the inertia of the apparatus coupled to it, is far too great to allow the parts to respond to impulses at a frequency of c./s., the motor remains stationary. This is the condition which obtains when the outline is a straight line and the scanning line of the photoelectric cell is truly transverse thereto. Under these conditions the tracer wheel is parallel to the outline and is rotating at a steady speed so that the upper carriage is being carried forward along a path which is parallel to the outline.

If the outline deviates then the photoelectric cell at once sees more light on one side of the outline than on the other, so that the impulses supplied to the amplifier become alternately larger and smaller and the amplifier faithfully reproduces these signals. The condition now is that the motor still receives alternate impulses tending to drive it in one direction and intervening impulses which tend to drive it in the other direction, but the impulses tending to drive it in one of the directions have become larger while the others have become smaller, so that the steering motor begins to rotate in the one direction. The mechanism is so arranged that the steering motor rotates both the shaft carrying the tracer wheel and the photoelectric device in such a direction as to steer the upper carriage in the direction in which the outline has deviated.

In order to give the steering device time to respond to deviations in the outline it is arranged that the photoelectric tracer device is so mounted that it is scanning a portion of the outline in advance of the axis of the scanning device. This may be achieved either by mounting the vibrating photoelectric cell arm off centre with respect to the axis about which the device rotates, so that it swings in an arc, or by mounting it centrally but at an angle to the vertical so that it looks ahead.

In order to improve the operation of the line follower it is common practice to include a small generator of the type known variously as an induction generator, a rate generator or a tacho-generator, which produces a voltage which varies strictly in accordance with its speed, i.e. it, produces a velocity signal. This is mechanically coupled to the steering motor and electrically connected in the circuitry in such a way as to oppose the signals from the photoelectric cell. The velocity signal stabilizes t the operation of the steering motor and prevents overshooting. The type of servo motor described and the use of the tacho-generator are so well known in ordinary servo practice that no further description is deemed necessary.

It will be obvious that, as the portion of the outline being viewed or scanned is in advance of the control mechanism, that is, the point of contact between the tracer wheel and the fixed portion of the wheel which it engages, errors will occur on curves which are related 1 in magnitude to the degree of curvature and are a maximum at sharp corners, since the following device will cause movement of the steering motor before the tracing wheel has reached the point at which it should be steered. When negotiating a right-angled sharp corner the follower mechanism will produce a rounded corner, the radius of the rounded corner depending upon the amount of lead and the characteristics of the mechanism. On the other hand, if the amount of lead is reduced below a certain point, instability in steering is likely to develop.

One object of the invention is to provide a method of following an outline in an oxygen jet cutting machine which is highly efficient and accurate.

Another object is to provide a method of following an outline which includes deriving a signal dependent upon the rate at which the outline deviates from the instantaneous direction of the following device, the signal being used to reduce the lead of the following device. A further object of the invention is to provide a following device for an oxygen jet cutting machine which will provide a high degree of accuracy of following on curves.

Still another object is to provide a following device for an oxygen jet cutting machine in which the portion of the outline being viewed or scanned is normally in advance of the relative position of the control mechanism and the amount of the advance or lead is reduced according to the rate of deviation of the outline from the instantaneous direction of the following device.

Further objects and advantages of the invention will appear from the description thereof which follows.

To promote a full understanding of the invention selected embodiments thereof will now be described by way of non-limiting examples. In the drawings:

FIGURE 1 is a diagrammatic representation of an oxygen jet flame cutting machine of a known kind showing a photoelectric outline following device;

FIGURE 2 is an elevation of one form of following device showing the application of the invention;

FIGURE 3 is a plan view of the device of FIGURE 2;

FIGURE 4 is a sectional elevation of an electro-magnetic actuator used in the device of FIGURES 2 and 3;

FIGURE 5 is a cross-sectional elevation of the actuator of FIGURE 4;

FIGURE 6 is a circuit diagram showing the connections to the electro-magnetic actuator of FIGURES 4 and 5;

FIGURE 7 is a diagram showing the action of the device of FIGURES 2 and 3;

FIGURE 8 is an elevation of a following device similar to that of FIGURE 2 showing another application of the invention;

FIGURE 9 is a plan view of the device of FIGURE 9;

FIGURE 10 is a diagram showing the operation of the device of FIGURES 8 and 9;

FIGURE 11 is an elevation of a following device showing a further application of the invention;

FIGURE 12 is a plan view of the device of FIGURE 11; and

FIGURE 13 is a diagram of the operation of the device of FIGURES 11 and 12.

Referring to FIGURE 1, a flame cutting machine suitable for use with the invention comprises a framework 11 provided with a pair of rails 12. A lower carriage 13 is provided with pairs of wheels 14 on each side by which it may run along the length of the rails 12. The movement of the lower carriage 13 is thus normal to the plane of the drawing of FIGURE 1. The lower carriage 13 is provided on its upper surface with a further pair of rails 15 set transversely of the rails 12. An upper carriage 16 has wheels 17 adapted to run on the rails 15. Thus, the upper carriage 16 is movable in any direction in a plane parallel to the ground plane on which the frame 11 stands. A support 18 mounted on the upper carriage 16 carries a rotatable photoelectric following device 19, including a photoelectric cell 19a, which may be of a known kind; it may, for example, consist of an arrangement in which an image of a part of the thickness of a thick line is projected on to the photoelectric cell from which the signal falls below a mean level if the amount of light falling on it is below a predetermined level and from which the signal rises about the mean level if the light projected on to it is above this level. These charges are caused to operate steering mechanism so that the device will always tend to keep itself in such a position that the spot projected on to it consists of a semi-circle of the dark outline and a semi-circle of the white background. Alternatively it may consist of a vibratory mechanism in which either the spot of light used for scanning, or the photoelectric element itself, is vibrated, two signals being delivered, one corresponding to the amount of light seen on one side of the outline and the other corresponding to the amount of light seen on the other side of the outline, the steering mechanism being operated in such a manner as always to maintain a balance between the two signals. Assuming that the follower is of the type in which the photoelectric cell 19a is vibrated, then it will be attached to an arm which is coupled to a vibrator. The area of the table below the follower 19 is brightly illuminated, for example, by a spot light (not shown) and an image of a part of the outline is projected upwards by a lens 31 on to the photoelectric cell. Due to the vibration of the cell it alternately looks at one side and the other side of the image. If the image is at the centre of the vibratory path of the cell then the cell output rises to the same point above a set reference level when looking on each side of the image. If, however, the image is not central then the cell output rises to a higher level on one side of the image than on the other. The alternate signals are compared and as soon as one of them rises above the level of the other a dilference signal is generated and is used to control an amplifier which causes the steering motor to run in one direction or the other, in dependence upon the sense of the signal. The framework 11 includes a tracing table 20 lying in a horizontal plane and the surface thereof is frictionally engaged by tracer wheel 21 which is driven at a set speed through appropriate gearing by a motor contained in a rotatable housing 22. The rotatable following device 19 and the rotatable housing 22 are coupled together by means of a chain 23 engaged in sprockets respectively on the device 19 and the housing 22. The steering motor 24 rotates the device 19 and the housing 22 synchronously through the chain drive. A rate generator or tacho-generator driven by the steering motor generates a signal which is connected in circuit to oppose the signal from the amplifier.

The arran ement so far described is known. According to a further feature which has been proposed allowance is made for the kerf or the width of the cut made by the oxygen jet cutter, by means of an optical deviator 25. It will be appreciated that if the photoelectric tracing device follows the outline in the normal way then the flame cutter will follow a path which corresponds with the outline but since the cut made by the flame cutter has a definite width the object finally cut from the plate will be slightly smaller than the outline all around its periphery. This may be overcome by making the original outline slightly larger in all directions but the diificulties involved are avoided by using the optical deviator. This consists of a block of glass which is arranged to deflect the image received by the photocell in a direction transverse to the line of movement. By suitable adjustment of the angle of tilt of the deviator allowance may be made for the kerf or, if desired, the object cut from the plate may be made larger or smaller than the outline.

Fixed to the upper carriage 16 is a bar 26 which is provided at its end with a supporting arm 27 for an oxygen jet cutting burner 28. Supporting members 29 are provided at the side of the framework 11 to support a sheet of material 30 from which an object is to be out.

In operation the tracer wheel 21 is rotated at a predetermined speed and causes the upper carriage 16 to move in a direction lying along the plane of the tracer wheel 21. At the same time an image of a part of the outline is projected through a lens 31 and the deviator 25 to the following device 19. If the portion of the outline being followed is a straight line and the plane of the tracer wheel 21 is parallel with this line then the tracer wheel will continue to move in that direction, pulling the carriage 16, the cutter 28 and the device 19 with it. If, however, the outline deviates from the straight path then the deviation causes the following device 19 to provide the appropriate signal through the amplifier to the steering motor 24 to rotate the device 19 about its axis in such a manner that it tends to continue to follow the deviating outline. Rotation of the device 19 simultaneously rotates the housing 22, thus steering the tracer wheel 21. The tracer wheel 21 alters the direction of movement of the upper carriage 16, and enables the device 19 to continue to follow the outline. The change in the direction of the upper carriage 16 causes a corresponding change in the direction of movement of the cutting head 28, so that the cutting head 28 eventually cuts a shape corresponding to the shape of the outline.

In order to allow time for a deviation of the outline detected by the tracer head to operate the steering mechanism and to change direction of movement of the upper carriage 16 it must be arranged that the portion of the outline being viewed or scanned by the device 19 is slightly in the lead in relation to the position of the tracer wheel 21 which constitutes the control mechanism. Thus, at any moment, the point on the outline on which the device 19 is working is slightly in front of the position of the tracer wheel 21.

The amount of lead must exceed a certain minimum or it is found that instability in steering occurs. Apart from the feature previously referred to, the jet cutting machine, the method of operation and the technical considerations so far described are known in the art. It will be evident that, due to the lead, an error will develop when a curve is being followed, the magnitude of the error increasing as the curve becomes sharper and reaching a maximum on a sharp corner, as the following device will cause a movement of the steering motor before the tracing wheel has reached the point where turning should have taken place. This error is eliminated or reduced by the following device according to the invention.

When, in the known device, a. straight line is being followed no deviation signal is produced by the follow ing device. Assuming now that the following device reaches the junction of two straight lines which form an obtuse angle then, as the junction is reached, a signal is produced which rises rapidly as the photoelectric device moves over the junction. The signal causes the steering motor to run in the appropriate direction to steer the follower on to the new line and as steering progresses the deviation signal dies away to zero. If, on the other hand, the follower is following a straight line which runs into a curve which is a part of a circle then a deviation signal is generated and the motor begins to run as before, but the signal does not dies away and, in fact, remains at a constant level, representing a constant rate of deviation, so that the motor will continue to run at a steady speed to provide a steady steering movement around the curve. It will, however, always lag slightly, as explained above. Thus, due to the restoring action of the steering mechanism, which is continually endeavouring to reduce the deviation to zero, the deviation signal becomes effectively dependent upon the rate of deviation.

In applying the invention a signal is required which is dependent upon the rate of deviation of the outline from the instantaneous direction of the device 19. When, in the known arrangement, the device 19 is following a straight line it provides no signal at all but as soon as the outline deviates the device 19 provides a signal which is dependent upon the rate of deviation. The speed of the steering motor 24 is substantially proportional to the signal from the device 19. A more powerful signal, also dependent upon the rate of deviation may be obtained from a tacho-generator coupled to the steering motor 24. A tacho-generator is also known as an induction generator or a rate generator. As has already been pointed out, the error in following increases as the sharpness of the curve increases and is a maximum when a sharp corner is reached. According to the invention the signal dependent upon the rate of deviation is used to reduce the lead of the device 19 over the control mechanism, i.e. the tracer wheel, and meanswill now be described for achieving this objective.

FIGURES 2 and 3 show the optical system of a following device comprising a housing 36 which is provided at its upper end with an element 37, which could be a stationary photoelectric cell, or an oscillating photoelectric cell, or a light source, according to the particular photoelectric system which is employed. The lower part of the housing 36 is provided with a pair of trunnions 38 and a casing 39 is supported on a shaft 40 and a stubshaft 41 in the trunnions 38, the common axis of the shaft and stub-shaft being indicated at 57. The casing 39 is adapted to rotate to a limited extent about the axis 57. Near its upper end the casing 39 contains a glass cylinder 42 which is rotatable about an axis 43 spaced from, and at right-angles to, the axis 57. At its lower end the casing 39 contains a lens 45 which projects an image of a portion of the outline 46 upwardly and on to the glass cylinder 42. As shown, the direction of movement of the following device is in a direction normal to the plane of the drawing. It will be evident that if the casing 39 is swung about the axis 57 -so that the lens 45 moves in the direction in which the following device as a whole is moving, the portion of the line 45 which is projected will be in advance of the portion of the line lying under the axis of the housing 36. In other words, the following device will lead the control mechanism.

The shaft 40 has a skew gear 47 attached to its outer end and this is engaged by another skew gear- 48 on' th e success I end of a shaft 49 of the rotor b of an electro-magnetic' actuator comprising a stator 51. The electric-magnetic actuator is carried on an arm 6i; it is illustrated in more detail in FIGURES 4 and 5. Referring to those figures, the electro-magnetic device comprises the stator 51 which carries a suitable winding adapted, when energized with direct current, to set up a field across its diameter passing through the axis of the shaft 49. The rotor 50 may be of soft iron and may have a winding but is preferably a permanent magnet with its poles at two diametrically opposite points, as indicated by the latters N and S in FIGURE 4. At the end of the shaft 49 remote from the gear 48 a spring 52 has one end anchored to the shaft 49 and its other end anchored at 53 to the stator 51. The spring 52 is wound up to a predetermined tension and is prevented from unwinding by a bar 56 engaging one of two stop screws 54 and 55. The field set up by the winding of the stator 51 is normally out of line with the field of the permanent magnet of the rotor 50, so that if the winding 51 is energized with direct current flowing in the appropriate direction a turning moment is exerted on the rotor 50 as its magnetic poles endeavor to line themselves up with the field set up by the stator winding. This causes the rotor to turn so that the bar 56 moves away from the stop screw against which it has been resting and moves towards the other stop screw. Rotation of the rotor 50 rotates the shaft 40 through the gears 48 and 47 and thus swings the casing 39 about the axis 57. The mechanism is so arranged that the spring 52 normally holds the bar 56 against the stop screw 54 and this swings the casing 39 in the direction to give the maximum lead. While the tracer mechanism is following a straight line the steering motor is stationary and the maximum lead is maintained. As soon as the outline begins to deviate slightly the steering motor begins to run and the nacho-generator generates a voltage which is related to the amount of curvature of the outline which has been encountered. FIGURE 6 shows the connections of the tacho-generator to the electro-magnetic actuator. The tacho-generator field 70 is excited by a constant DC. voltage and its armature 71 is connected across a potentiometer 72. A part of the armature voltage is tapped olf by the slider 73 and is applied to the winding 74 of the stator 51 and the field set up thereby acts upon the magnetic rotor 50. The voltage applied to the windings of the stator 51 causes some rotation of the rotor 50 against the tension of the spring 52. As the result the casing 39 is swung back to some extent, to reduce the amount of lead. If the deviation of the outline increases the output of the tacho-generator increases and the rotor 50 experiences a more powerful torque, so that it turns a little further against the tension of the spring 52, thus further reducing the lead. If the outline contains a very sharp curve or a corner then the steering motor increases its speed up to a maximum, thus giving a maximum output voltage from the tacho-generator and maximum torque on the rotor 50, which is rotated through the maximum angle permitted by the stop screw 55. It is arranged that under this condition the casing 39 is swung into, or almost into, a vertical position so that the lead is reduced to a very small amount or to zero. As soon as the tracer mechanism has negotiated the sharp curve or corner the curvature of the outline is either reduced or changed to a straight line. The speed of the steering motor is then reduced either to an intermediate speed or to zero and the output of the tacho-generator follows. The spring 52 now overcomes the torque of the rotor 50 and moves it either partly or wholly back to the stop screw 54 so that the lead of the following device is either partly or wholly re-established.

FIGURE 7 shows diagrammatically the operation of the arrangement shown in FIGURES 2 and 3. When the casing is swung about the axis 57 under the influence of the spring 52 the axis of the optical system 58 is swung forward with respect tothe vertical axis 59 to the extent indicated by the dimension 60. When a curve is reached the rotation of the rotor under the influence of the signal from the tacho-generator causes the casing 39 to swing backwards so that the axis 58 begins to approach the axis 59 and the dimension 60 is reduced or eliminated.

The glass cylinder 42 when in its normal position as shown in FIGURES 2 and 3, will allow the image of the line 45 to pass directly upwards through it to the element 37. It may, however, be swung in one direction or the other on the axis 43 by means of a graduated knob 62. After it is moved from its central position it causes the image of the line 46 to be displaced to one side or the other so that the following device will then run on one side or the other of the line 45. This enables allowance to be made for the kerf or, if desired, enables the piece cut from the workpiece to be made larger or smaller than the outline. The adjustment of the knob 62 is quite independent of the operation of the lead adjusting device previously described.

FIGURES 8, 9 and 10 show an alternative arrangement. In these figures elements corresponding with elements shown in FIGURES 2, 3 and 7 have been given like reference numbers. The casing 39, instead of being mounted on the shaft 4% and stub shaft 41 to enable it to swing, is now provided with lugs 63 and 64 projecting from opposite sides, the lugs being bored so that they may slide on two bars respectively 65 and 66 inside the housing 36. The electro-magnetic device comprising the rotor 50 and the stator 51 is of similar construction but is now mounted with its axis horizontal and has a toothed pinion 67 mounted on the end of its shaft 49. The pinion 67 engages a rack 68 attached to the casing 39. Thus, the casing 39, instead of being swung to and fro about an axis, is now bodily moved forwards or backwards along the line on which the tracer mechanism itself moves.

As shown in FIGURE 10 the lead, indicated by the dimension 60, is now equal to the amount by which the casing 39 is moved forward with respect to the vertical axis 59.

FIGURES 11, 12 and 13 show a further method of carrying the invention into effect. In this embodiment the housing 36 is not required and the casing 39, of larger diameter, is fixed. The glass cylinder 42 is carried in gimbals so that it may be swung about two axes mutually at right angles, and at right-angles to the vertical axis 59. The electro-magnetic device comprising the rotor 50 and stator 51 is mounted with its axis vertical as in FIGURE 2 and has a gear 48 secured to its shaft 49 engaging a gear 47 as before. The gear 47 is carried on a shaft 69 which carries a frame in which the glass cylinder 42 is supported. As shown in FIGURE 13 the lead is now given by swinging the glass cylinder 42 about the axis 7!) of the shaft 69 and the amount by which it is swung forward gives a lead, indicated by the dimension 60. To provide for the lateral displacement to allow for kerf, or enlargement or reduction of the size of the cut piece with respect to the outline, the glass cylinder 42 is rotated by means of the knob 62 about the axis 43 as in FIGURES 2 and 8. In the last embodiment the caisng is stationary and the glass cylinder 42 is given a compound movement composed of a variable swing about the first axis 70 to provide a variable lead and a presettable swing about the second axis 43 at right-angles thereto to provide for kerf adjustment.

I claim:

1. In an oxygen jet cutting machine containing a photoelectric following device which leads a control mechanism, means to derive a signal dependent upon the amount by which an outline being followed deviates from the instantaneous direction of said following device, and electr c-magnetic means responsive to said signal for automatically reducing the amount of lead of said following device in accordance with the amount of deviation.

2. An arrangement as claimed in claim 1 comprising a steering motor adapted to change the path of said following device, and a tacho-generator driven by said steering motor to generate said signal in dependence upon the speed of said steering motor.

3. An arrangement as claimed in claim 1 comprising an electro-magnetic actuator including a wound stator to which said signal is applied, a magnetic rotor acted upon by the stator field and adapted to turn about its axis to reduce the amount of lead, and a spring to unge said rotor to the position of maximum lead, said rotor rotating in respense to said signal against the tension of said spring.

4. An arrangement as claimed in claim 1 comprising an optical system including an optical deviator associated with said following device, said optical deviator being movable about a first axis transverse to the line of travel of said following device in response to said signal to reduce said lead, and being also adjustable about a second axis at right-angles to said first axis to cause transverse displacement of the image transmitted to said following device.

5. An oxygen jet cutting machine comprising a carriage movable in all directions in a horizontal plane, an oxygen jet cutter on said carriage movable therewith, a horizontal plane support for an outline, a photoelectric following device on said carriage adapted to produce steering signals, a power driven steerable tracer wheel engaging the surface of said support and adapted to move said carriage, a steering motor to steer said tracer wheel in response to said steering signals, generator means driven by said motor to generate a signal dependent upon the speed of said steering motor, an optical system associated with said following device and having a lead with respect thereto, and means to move at least a part of said optical system in response to the said signals for automatically reducing the lead of said following device in dependence upon the speed of said motor.

6. A machine as claimed in claim 5 comprising an electro-magnetic actuator including a wound stator fed with said signal, a magnetic rotor acted upon by the field of said stator adapted to turn about its axis to move at least a part of said optical system, and a spring to urge said rotor to the position of maximum lead, said rotor rotating in response to said signal against the tension of said spring to reduce said lead.

7. A machine as claimed in claim 6 wherein said optical system includes an optical deviator, said optical deviator is movable about a first aXis transverse to the line of travel of said following device in response to said signal to reduce said lead, and is also adjustable about a second axis at right-angles to said first axis to cause transverse displacement of the image transmitted to said following device.

References Cited in the file of this patent UNITED STATES PATENTS 2,261,644 Cockrell Nov. 4, 1941 2,419,641 Hart Apr. 29, 1947 2,499,178 Berry et al. Feb. 28, 1950 2,851,643 Limberger Sept. 9, 1958 2,868,993 Henry Jan. 13, 1959 

